1. Field of the Invention
The present invention relates to a fixing apparatus configured to fix a toner image onto a sheet and an image forming apparatus including the fixing unit.
2. Description of the Related Art
Image forming apparatuses configured to form an image using an electrophotographic method includes a fixing unit configured to fix a toner image onto a surface of a recording material (referred to as sheet or paper) by a transfer method or a direct method. When the fixing unit fixes the toner image, paper with the unfixed toner image is guided to pass through a nip portion formed by a pair of rollers (i.e., a heating roller with a heated surface and a pressure roller). While the paper passes through the nip portion, heat and pressure is applied to the sheet. Accordingly, the unfixed toner is fused and fixed to the surface of the paper.
Since the unfixed toner on the paper is fused when the paper passes through the nip portion, there has been a possibility that a jam is caused by the paper wrapping around the heating roller due to the viscous behavior of the toner. Conventionally, in order to avoid such a paper jam, the fixing unit, such as the one described above, uses a separation claw which contacts the heating roller to separate the paper wrapping around the heating roller.
The separation claw can easily separate the paper from the heating roller if the image density is low or if enough space is provided between the leading edge of the paper and the top of the image (hereinafter referred to as a margin). Meanwhile, for example, if paper of a small grammage (especially coated paper) is used or the toner image is a color image of increased density or, further, the margin of the paper is small, the paper tends to wrap around the heating roller. Thus, in such a case, the separation claw may not be able to appropriately separate the paper from the heating roller.
Further, since the separation claw contacts the heating roller, the claw may scratch the surface of the heating roller. In such a case, the scratch may be transferred to the fused toner and a defective image may be formed.
Under such circumstances, a separation method that does not form a scratch on the heating roller is discussed in Japanese Patent Application Laid-Open No. 2004-212954 and No. 2009-271345. According to the separation method, compressed air is jetted from a nozzle toward the heating roller to separate the paper from the heating roller. Further, according to the method discussed in Japanese Patent Application Laid-Open No. 2004-212954, air compressed by a pump is jetted from a solenoid valve in pulses to separate the paper.
However, as discussed in Japanese Patent Application Laid-Open No. 2004-212954 and No. 2009-271345, since the air compressed by a pump is blown out from a nozzle, if the solenoid valve fails due to, for example, its deterioration, it may result in a paper jam at the fixing unit. If the paper jam occurs due to a failed solenoid valve, processing for solving the jam takes time, which results in the downtime of the image forming operation. Thus, it is convenient if the failure of the solenoid valve can be detected in advance.